Non invasive monitoring and imaging using non-ionizing radiation, allows medical professionals to diagnose and monitor a patient without invasive surgeries, or even without drawing blood. Pulse oximetry is one such revolutionizing technology, where non invasive monitoring of blood oxygenation using light has replaced blood gas analysis. Thus, pulse oximetry has become a gold standard monitor in every clinical setting, and has saved millions of lives.
During non-invasive monitoring, the concentration of certain chromophores (such as oxygenated and deoxygenated hemoglobin in oximetry) is calculated by detecting light that escapes the tissue, determining the optical properties of the tissue, and deriving therefrom the concentrations of the chromophores. Providing the tissue is homogenous, simple models allow for the calculation of these concentrations. However, as biological tissue is a complex scattering medium, measuring the local optical properties becomes a challenging task.
As light is highly scattered while propagating through turbid media such as biological tissue, photons that escape the tissue and reach a detector do not provide information about the path that they followed as they propagated through the medium. To acquire information about the optical properties of the tissue in the photons' path, several methods and algorithms have been developed. Such methods include frequency-domain spectroscopy, and photoacoustic spectroscopy [D M Hueber et al Phys. Med. Biol. 46 (2001) 41-62].